In recent years, in view of savings in resources and conservation of energy, there have been increasing demands for nickel-hydrogen secondary batteries, lithium ion secondary batteries, etc. which can be used repeatedly. Among them, lithium ion secondary batteries are characterized by lightness in weight, high electromotive force, and high energy density. Thus, there are growing demands for the lithium ion secondary batteries as power sources for driving various kinds of portable electronic devices and mobile communication devices such as mobile phones, digital cameras, video cameras, and lap top computers.
On the other hand, to reduce used amount of fossil fuel, and to reduce the amount of emission of CO2, expectations for secondary batteries are growing to serve as power sources for driving motors such as vehicles.
A battery pack which is a set of such secondary batteries includes a plurality of batteries in order to obtain a preferable voltage and battery capacity, and has a configuration in which a plurality of battery blocks are provided, where each battery block includes the plurality of batteries connected to each other in parallel and/or in series. Such a battery block is described in, for example, Patent Document 1.